U.S. patent number 10,987,760 [Application Number 15/164,435] was granted by the patent office on 2021-04-27 for method of manufacturing a holding plate, in particular for a clamp for holding wafers.
This patent grant is currently assigned to BERLINER GLAS KGAA HERBERT KUBATZ GMBH & CO.. The grantee listed for this patent is Berliner Glas KGaA Herbert Kubatz GmbH & Co.. Invention is credited to Mike Fischer, Ralf Hammer, Gregor Hasper, Stefan Hoescheler.
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United States Patent |
10,987,760 |
Hammer , et al. |
April 27, 2021 |
Method of manufacturing a holding plate, in particular for a clamp
for holding wafers
Abstract
Method of manufacturing holding plate (11) including ceramic
material of several chemical elements and configured for holding
apparatus (10) for holding a component by electrostatic forces or
vacuum, includes the steps of material removal from holding plate
(11) by laser ablation, wherein by laser irradiation (1)
protrusions (13) are formed on holding plate (11), end faces (13.1)
of which span a carrier surface for the component, and surface
modification of holding plate (11) by laser irradiation (1),
wherein irradiation parameters of laser irradiation (1) are set
such that at least one of the chemical elements of the ceramic
material is enriched on the surface of holding plate (11). A method
is also described of manufacturing holding apparatus (10) for
holding a component by electrostatic forces or vacuum, holding
plate (11) which is produced with the inventive method, and holding
apparatus (10) with at least one holding plate (11).
Inventors: |
Hammer; Ralf (Freiberg,
DE), Hoescheler; Stefan (Berlin, DE),
Fischer; Mike (Berlin, DE), Hasper; Gregor
(Berlin, DE) |
Applicant: |
Name |
City |
State |
Country |
Type |
Berliner Glas KGaA Herbert Kubatz GmbH & Co. |
Berlin |
N/A |
DE |
|
|
Assignee: |
BERLINER GLAS KGAA HERBERT KUBATZ
GMBH & CO. (Berlin, DE)
|
Family
ID: |
1000005513351 |
Appl.
No.: |
15/164,435 |
Filed: |
May 25, 2016 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20160354864 A1 |
Dec 8, 2016 |
|
Foreign Application Priority Data
|
|
|
|
|
Jun 3, 2015 [DE] |
|
|
102015007216.1 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B23K
26/352 (20151001); H01L 21/6833 (20130101); H01L
21/6838 (20130101); B23K 26/402 (20130101); B23K
26/0622 (20151001); B23K 26/0884 (20130101); B23K
37/0235 (20130101); B23K 26/361 (20151001); B23K
26/0006 (20130101); B23K 26/355 (20180801); B23K
2103/52 (20180801) |
Current International
Class: |
B23K
26/08 (20140101); B23K 26/0622 (20140101); B23K
26/00 (20140101); B23K 26/402 (20140101); B23K
26/352 (20140101); B23K 26/361 (20140101); H01L
21/683 (20060101); B23K 37/02 (20060101) |
Field of
Search: |
;264/400 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
4119254 |
|
Dec 1992 |
|
DE |
|
4119878 |
|
Dec 1992 |
|
DE |
|
4416479 |
|
Nov 1995 |
|
DE |
|
0669298 |
|
Aug 1995 |
|
EP |
|
59006303 |
|
Jan 1984 |
|
JP |
|
2012206137 |
|
Oct 2012 |
|
JP |
|
2011084531 |
|
Jul 2011 |
|
WO |
|
2013156236 |
|
Oct 2013 |
|
WO |
|
Other References
Lasered Substrate Chamfering and Beveling, Valley Design
Corporation,
https://web.archive.org/web/20140315160840/http://www.valleydesign.com/la-
ser-drilling.htm, Published Mar. 15, 2014 (Year: 2014). cited by
examiner .
Machine Translation of JP 59-6303 (Year: 1984). cited by examiner
.
Machine Translation of JP 2012206137 (Year: 2012). cited by
examiner .
Defintion of Proportion, Merriam-Webster,
https://www.merriam-webster.com/dictionary/proportion (Year: 2019).
cited by examiner .
Zerodur Expansion Glass Ceramic, Schott AG,
https://www.schott.eom/d/advanced_optics/f7ae3c11-0226-4808-90c7-59d6c881-
6daf/1.3/schott_zerodur_katalog_july_2011_en.pdf (Year: 2011).
cited by examiner .
English language abstract for DE 4119254 A1 (1992). cited by
applicant .
English language abstract for DE 4119878 A1 (1992). cited by
applicant .
P. Rudolph, "Micro-Ramanspectroscopy with Siliziumcarbid" in
Dissertation "Physical Chemistry of lasermaterial-interaction with
Ba-Al-Borosilikatglass, AIN, Sic, SiC-TiC-TiB2" (Berlin 2001), pp.
75-78. cited by applicant .
Search Report from corresponding GB16094823 dated Oct. 31, 2016.
cited by applicant .
Meijer, "Laser beam machining (LBM), state of the art and new
opportunities", Journal of Materials Processing Technology 149, pp.
2-17 (2004). cited by applicant .
Temmler et al., "Design Surfaces by Laser Remelting", Physics
Procedia 12(A), Lasers in Manufacturing 2011--Proceedings of the
Sixth International WLT Conference on Lasers in Manufacturing, pp.
419-430 (2011). cited by applicant .
Yeo et al., "Surface Roughness Changes in Al2O3 Induced by Nd:YAG
Laser Irradiation", Journal of the Korean Physical Society 59(2),
pp. 666-669 (2011). cited by applicant .
English language abstract of EP 0686612 A1 (1995). cited by
applicant .
Bonse, J.O. (2001). Chapter 7, Summary and Outlook. In "Material
processing of semi-conductors and nitride ceramics with ultrashort
laser pulses", Technical University, Berlin, Germany, pp. 122-125.
cited by applicant.
|
Primary Examiner: Minskey; Jacob T
Assistant Examiner: Sparks; Russell E
Attorney, Agent or Firm: Caesar Rivise, PC
Claims
What is claimed is:
1. A method of manufacturing of a holding plate having a ceramic
material of one single phase with at least two chemical elements
and being configured for a holding apparatus for holding a
component by electrostatic forces or vacuum, the method comprising:
material removal from the holding plate by laser ablation, wherein
by a laser irradiation a plurality of protrusions are formed on the
holding plate, end faces of which span a carrier surface for the
component, and surface modification of the holding plate by the
laser irradiation, wherein irradiation parameters of the laser
irradiation are set such that at least one of the chemical elements
of the ceramic material is enriched on the surface of the holding
plate such that the stoichiometric ratio of the chemical elements
on the surface, in comparison with the volume material, is shifted
towards at least one of the chemical elements, the enrichment
including the formation of a surface layer which includes said at
least two chemical elements and, by deviation from the volume
material, has a higher proportion of at least one of the chemical
elements relative to remaining chemical elements of said at least
two chemical elements within said surface layer.
2. The method according to claim 1, wherein a gloss is formed on
the surface of the holding plate by the at least one enriched
chemical element.
3. The method according to claim 1, wherein the at least one of the
chemical elements is enriched exclusively on the surface of the
protrusions.
4. The method according to claim 3, wherein the at least one of the
chemical elements is enriched exclusively on lateral surfaces of
the protrusions.
5. The method according to claim 1, wherein the material removal
takes place in layers in a plurality of irradiation steps, wherein
in each irradiation step, a layer of the ceramic material is
removed corresponding to a predefined irradiation pattern of the
laser irradiation.
6. The method according to claim 5, wherein in each irradiation
step, the irradiation pattern of the laser irradiation is changed
and the protrusions are formed with a predefined lateral
contour.
7. The method according to claim 6, wherein in each irradiation
step, the irradiation pattern of the laser irradiation is changed
and the protrusions are formed with at least one of rounded edge
portions and rounded base portions.
8. The method according to claim 1, wherein, except for the
protrusions, the material removal takes place across the surface
with an aspect ratio such that the end faces comprise less than 10%
of a total surface area exposed to the laser irradiation.
9. The method according to claim 8, wherein, except for the
protrusions, the material removal takes place across the surface
with an aspect ratio such that the end faces comprise less than 5%
of the total surface area exposed to the laser irradiation.
10. The method according to claim 1, wherein: the laser irradiation
is generated with a laser source which is moved relative to the
holding plate, the laser source in communication with a carrier
head which can be moved along at least three axes, the laser
irradiation takes place successively in a plurality of surface
regions of the holding plate, an extension of each of which is less
than a maximum working area of the carrier head, and the carrier
head and the holding plate are moved relative to each other between
the irradiations of the surface regions such that the surface
regions overlap.
11. The method according to claim 1, wherein the ceramic material
comprises SiC, Si.sub.2C, Si.sub.3N.sub.4, CrN, WC, B.sub.4C, AlN
or Al.sub.2O.sub.3.
12. The method according to claim 11, wherein the ceramic material
comprises Si.sub.3N.sub.4, SiC or Si.sub.2C, and the irradiation
parameters of the laser irradiation are set such that surfaces of
the holding plate have a higher Si proportion than a volume
material.
13. The method according to claim 1, wherein the protrusions have
at least one of: burls having a cylindrical, truncated pyramid or
frustoconical form, burls with an end face with a diameter of less
than 300 .mu.m, burls with a height of more than 25 .mu.m, burls
which are connected to the holding plate via a base portion which
has a rounded casing contour, and at least one of webs and
ribs.
14. The method according to claim 1, wherein the laser irradiation
has at least one of: laser pulses with a pulse duration in a range
of 2 ns to 300 ns, laser pulses with a repetition rate in a range
of 30 kHz to 200 kHz, and a wavelength at which absorptions of
phases of the ceramic material overlap to a maximum.
15. The method according to claim 1, wherein another process of
material removal after laser ablation additionally comprises a
removal of volume material between the protrusions by at least one
of a mechanically acting tool and sink erosion.
16. A method of manufacturing of a holding plate having a ceramic
material of a plurality of phases with at least two chemical
elements and being configured for a holding apparatus for holding a
component by electrostatic forces or vacuum, the method comprising:
material removal from the holding plate by laser ablation, wherein
by a laser irradiation a plurality of protrusions are formed on the
holding plate, end faces of which span a carrier surface for the
component, and surface modification of the holding plate by the
laser irradiation, wherein irradiation parameters of the laser
irradiation are set such that at least one of the chemical elements
of the ceramic material is enriched on the surface of the holding
plate such that the stoichiometric ratio of the chemical elements
on the surface, in comparison with the volume material, is shifted
towards at least one of the chemical elements, wherein the
enrichment includes the formation of a surface layer which includes
said plurality of phases and, by deviation from the volume
material, has a higher proportion of at least one of the phases
relative to remaining phases of said plurality of phases within
said surface layer.
17. A method of manufacturing a holding apparatus for holding a
component by electrostatic forces or vacuum, wherein at least one
holding plate of the holding apparatus is produced with the method
according to claim 1.
Description
BACKGROUND OF THE INVENTION
The invention concerns a method of manufacturing a holding plate
which is provided for a holding apparatus for holding a component
by electrostatic forces or vacuum, in particular a method of
manufacturing a holding plate which is adapted for holding a
semiconductor component, such as for example a wafer, with
electrostatic forces or by means of vacuum, and which has a
structured surface with protrusions, e.g. protruding burls, formed
by material removal. The invention furthermore concerns a method of
manufacturing a holding apparatus which is provided with the
holding plate. The invention furthermore concerns a holding plate
of a ceramic material with a structured surface with protrusions,
which is configured for holding a component, and a holding
apparatus with such a holding plate, in particular an electrostatic
clamp or a vacuum clamp. Applications of the invention lie e.g. in
the manufacturing of tools, machines or wafers for the
semiconductor industry.
It is generally known to produce integrated circuits (semiconductor
chips) by a lithographic processing of semiconductor wafers which
are held during or between individual process steps with a holding
apparatus (clamp) by electrostatic forces (electrostatic clamp) or
by means of vacuum (vacuum clamp). Further uses of holding
apparatuses lie in inspection or measurement processes in which
e.g. the planarity of a wafer is detected.
The holding apparatus comprises one or two holding plates with
exposed surfaces for receiving semiconductor wafers, and electrode
devices, cooling devices and where applicable further electrical,
mechanical or pneumatic components. As integration density grows,
extremely high requirements are imposed for the planarity of the
semiconductor wafers, for example in exposure steps, which must be
guaranteed by the mechanical stability and planarity of the
surface(s) of the holding plate(s).
Since semiconductor wafers can be disruptively deformed simply by
dust particles on the surface, it has proved advantageous to
provide the holding plate with a plurality of protrusions, the free
faces (end faces) of which span a flat carrier surface. The
protrusions comprise e.g. so-called burls. The burls offer the
advantage of minimizing the contact area between the holding plate
and the semiconductor wafer, and eventually occurring dust
particles can be collected in the gaps between the burls.
Furthermore, it has proved advantageous in practice to manufacture
the holding plate from a ceramic material, since ceramic materials
are available with particularly high mechanical stability and
strength.
In the manufacturing of a conventional holding apparatus, firstly
the holding plate is made from the ceramic material with a free
flat surface. Then the burls are formed by material removal from
the ceramic material. Various removal methods are known for this,
such as for example material removal methods using a CNC milling
machine, or removal by means of sink erosion.
The conventional methods may have the following disadvantages, in
particular on manufacturing of burls with small diameters (e.g.
less than 500 .mu.m). Machining CNC processing may result in
undesirable flaking at edges of the protrusions or recesses, which
can lead to a reduced abrasion resistance and the occurrence of
microscopically small ceramic particles. Holding plates machined by
sink erosion also have a tendency towards the undesirable creation
of ceramic particles. Furthermore, damage can occur to the ceramic
material on sink erosion. Observations in practice show an
increased fracture susceptibility of the burls as the burl diameter
decreases. The fracture susceptibility is particularly critical
since, actually in a holding plate with several thousand burls, the
loss of just a few burls can lead to a malfunction of the holding
apparatus. In many practical applications, even a disappearing
error rate is required.
A further, generally known method for structuring solid body
surfaces is based on material removal by laser ablation. Under the
effect of a focused laser beam, the solid body is vaporized locally
to create a recess in the solid body surface. Laser ablation was
initially proposed for metals and has also recently been used in
machining ceramic materials (see for example DE 44 16 479 A1; J.
Meijer in "Journal of Materials Processing Technology", Vol. 149,
2004, p. 2-17; S. Yeo et al. in "Journal of the Korean Physical
Society", Vol. 59, 2011, p. 666-669; and A. Temmler et al. in
"Physics Procedia", Vol. 12, 2011, p. 419-430).
Laser ablation of ceramic material has previously been restricted
mainly to surface processing of micro-mechanical components, and in
particular for creating local depressions such as for example holes
or trenches in an otherwise closed surface. It is known from
practice that attempts to produce burls on holding plates of
ceramic material by means of laser ablation have delivered
unsatisfactory results only. It has for example been found that the
burls can have such a high fracture susceptibility that on normal
use of the holding apparatus, they break simply under the effect of
the clamping force on holding of a component.
The objective of the invention is to provide an improved method of
manufacturing of a holding plate for electrostatic holding or
vacuum holding of a component, with which the disadvantages of
conventional methods are avoided. The objective of the invention
lies in particular in producing a holding plate so that protrusions
can be formed which have an increased break strength both during
manufacturing of the holding plate and on use of the holding
apparatus, and that protrusions can be formed with reduced lateral
dimensions e.g. burls with reduced diameter, and/or that the
holding plate can be manufactured with a reduced error rate. The
objective of the invention is furthermore to provide an improved
holding plate with projecting protrusions for electrostatic holding
or vacuum holding of a component, with which the disadvantages of
conventional holding plates are avoided. The holding plate is to be
distinguished in particular by an increased break strength of the
protrusions, and increased abrasion resistance and/or a reduced
tendency towards the formation of ceramic particles.
These objectives are achieved by methods, a holding plate and a
holding apparatus of the invention.
DESCRIPTION OF THE INVENTION
According to a first general aspect of the invention, a method is
provided of manufacturing of a holding plate from a ceramic
material which is adapted for a holding apparatus for holding a
component by means of electrostatic forces or vacuum. The ceramic
material is composed of several (two or more) chemical elements.
The chemical elements form one or several (two or more) phases of
the ceramic material. Each phase comprises a region of uniform
chemical composition consisting of a single chemical element or a
single chemical compound with a plurality of chemical elements. On
at least one surface of the holding plate, a plurality of raised
protrusions is formed by material removal of the ceramic material.
The surface of each protrusion comprises an end face (surface at
the free end of the protrusion with a surface normal perpendicular
to the extension of the holding plate) and a lateral surface
(remaining surface of the protrusion with the exception of the end
face, including the edge bordering the end face, lateral side face
and base face). The end faces of the protrusions span a plane
carrier surface for the component to be held.
According to the invention, the material is removed by laser
ablation. The laser ablation comprises removal of the ceramic
material from the surface of the holding plate by pulsed laser
irradiation. The laser irradiation takes place locally selectively,
according to a geometric irradiation pattern, such that the ceramic
material of the protrusions to be formed is excluded from the laser
ablation.
According to the invention, furthermore--preferably after material
removal--a surface modification (surface machining) of the holding
plate takes place by means of the laser irradiation. Irradiation
parameters of the laser irradiation are set such that one of the
phases of the ceramic material is enriched on a surface of the
protrusions. The surface modification comprises a pulsed laser
irradiation with irradiation parameters which are modified in
comparison with the material removal. The enrichment takes place
such that the stoichiometric ratio of the chemical elements on the
surface, in comparison with the volume material, is shifted towards
at least one of the chemical elements. If the ceramic material
comprises one single phase with several chemical elements, the
enrichment comprises the formation of a surface layer which, by
deviation from the volume material, has a higher proportion of at
least one of the chemical elements. If the ceramic material
comprises several phases each with one or several chemical
elements, the enrichment comprises the formation of a surface layer
which, by deviation from the volume material, has a higher
proportion of at least one of the phases. Advantageously, the
enrichment of the at least one chemical element of the ceramic
material achieves a smoothing and mechanical stabilization of the
surface of the ceramic material, in particular the protrusions.
Suitable irradiation parameters for the surface modification, in
particular the pulse duration, repetition rate and/or wavelength of
the laser irradiation, are e.g. determined by reference tables,
preliminary experiments or thermodynamic simulations.
According to a second general aspect of the invention, a method is
proposed of manufacturing of a holding apparatus which is
configured for holding a component by electrostatic forces or by
vacuum, wherein at least one holding plate of the holding apparatus
is produced with the method according to the first general aspect
of the invention. Furthermore, the at least one holding plate is
connected to at least one of an electrode device, a cooling device,
a vacuum device and further electrical components, mechanical
components and pneumatic components.
According to a third general aspect of the invention, a holding
plate is provided which is made from a ceramic material with
several chemical elements and has a plurality of protrusions on a
surface. The holding plate is configured for a holding apparatus
for holding a component by electrostatic forces or vacuum.
According to the invention, on the surface of the holding plate, at
least one of the chemical elements of the ceramic material is
enriched in comparison with the distribution of the chemical
elements in the volume of the ceramic material. Preferably, the
holding plate is produced with a method according to the first
general aspect of the invention.
According to a fourth general aspect of the invention, a holding
apparatus is provided for holding a component by electrostatic
forces or by vacuum, which has at least one holding plate according
to the third general aspect of the invention. The holding apparatus
is an electrostatic holding apparatus or a vacuum holding apparatus
for holding components, in particular semiconductor wafers.
The invention is based on the following considerations of the
inventors. It has been found that the fracture susceptibility of
the burls on conventionally produced holding plates is caused by a
porous surface structure, comprising for example pores, cracks and
scars in the ceramic material. For example, on sink erosion of
SiSiC (Si-infiltrated SiC) with the phases Si and SiC, it was found
that a porous surface is formed. The formation of the undesirable
ceramic particles on the porous surface is promoted. By means of
scanning electron microscopy (SEM), it was found that the surface
of ceramic material machined by sink erosion has irregular
machining structures with characteristic sizes of at least 10
.mu.m.
In contrast, it was found that material removal on the surface of
the holding plate by means of laser ablation, and the enrichment of
at least one of the chemical elements of the ceramic material,
gives a smooth non-porous surface. SEM studies showed that the
laser ablation and surface modification according to the invention
provide the surface of the ceramic material with machining
structures with characteristic sizes in the lateral or depth
direction of less than 5 .mu.m, in particular less than 2 .mu.m,
such as for example 1 .mu.m or less.
It was furthermore found that the surface modification causes a
phase conversion of the ceramic material on its surface. The phase
conversion may comprise a melting of the surface with subsequent
solidification, a correction of surface defects (filling of
remaining pores) and/or removal of residual microstructures.
Furthermore, the phase enrichment can take place by thermal
decomposition of a chemical compound, wherein, with the irradiation
parameters selected according to the invention for the surface
modification according to the phase diagram of the ceramic
material, the decomposition products are present in different
aggregate states and one of the decomposition components is
deposited on or embedded in the microstructure close to the
surface, while the other decomposition component remains as solid
crust which is easy to clean away, or is vaporized or volatilized
into a gaseous compound. As a result, at least one of the chemical
elements, in particular at least one of the phases of the ceramic
material on the surface of the protrusions, is enriched, i.e. the
chemical elements and, for a multiphase ceramic material, the
phases of the ceramic material are arranged on the surface of the
protrusions with a stoichiometric ratio being different from the
volume properties of the ceramic material. The stoichiometric ratio
of the phases--such as for example, in a two-phase material such as
e.g. SiSiC, the phases Si and SiC--on the surface (stoichiometric
surface ratio) is shifted in favor of one phase, e.g. Si, compared
with the stoichiometric ratio of the phases in the ceramic material
(stoichiometric volume ratio).
Also, in a monophase material, such as e.g. SiC, a surface
enrichment of one of the elements such as e.g. Si can take place in
that the elements Si and C are thermally decomposed, wherein as a
result the structure close to the surface is enriched with Si,
leaving C as the crust which is easy to clean away or extracted as
a gaseous compound (e.g. CO.sub.2). The enrichment of a metal or
semiconductor phase of the ceramic material on the burl surface has
proved particularly advantageous for achieving a smooth burl
surface.
According to a preferred embodiment of the invention, for the
surface modification of the holding plate, the irradiation
parameters of the laser irradiation are set such that a gloss is
formed on the surface of the holding plate by the enriched phase.
In other words, the surface of the holding plate is at least partly
mirror-reflective. Advantageously, a holding plate with a glossy
surface has advantages with regard to the mechanical stability of
the protrusions and the cleaning of the holding plate.
According to a particularly preferred embodiment of the invention,
the laser irradiation for the surface modification of the holding
plate takes place such that the surface layer with the at least one
enriched chemical element is formed exclusively in precisely
defined surface regions, preferably on the lateral surfaces of the
protrusions. Restricting the surface modification to the side faces
of the protrusions offers advantages for the processing speed,
without leading to a deterioration in the stability of the
protrusions. Alternatively or additionally, a modification of the
surface of the holding plate may be provided between the
protrusions, i.e. on the bottom surface between the protrusions.
This may be advantageous for reducing sources of contamination or
for the visual appearance of the holding plate.
According to a preferred embodiment of the invention, the laser
ablation comprises several irradiation steps. In each irradiation
step, a layer of the ceramic material is removed under the effect
of the laser irradiation. The laser irradiation takes place with a
predefined irradiation pattern according to the desired form and
position of the protrusions. Removal of the material by layers has
the advantage that the irradiation parameters of the laser
irradiation can be optimized for a gentle material removal, and the
desired depth of the structuring (height of protrusions) may be
achieved by repeating the irradiation steps.
Each irradiation step may be carried out with the same irradiation
pattern so that, as a result, the external form of the protrusions
is determined by the boundaries of the irradiation region and the
form of the radiation field in the focus of the laser irradiation.
According to an alternative and preferred variant of the invention,
the irradiation steps may be executed with different irradiation
patterns. Preferably, the irradiation pattern in each further
irradiation step, i.e. on the removal of each further layer of the
ceramic material, is changed such that the protrusions are formed
with a predefined lateral contour (shape of the surfaces in the
axial direction of the protrusions). Preferably, the irradiation
pattern is changed such that the edges of the lateral surface of
the protrusions towards the end faces and/or the base faces of the
protrusions are rounded. Advantageously, the rounded contour--in
particular in the base region of the burls--allows an improved
dissipation of mechanical stresses so that the break strength of
the burls is increased.
According to a further preferred embodiment of the invention, the
laser irradiation is directed serially in points or lines onto the
surface of the holding plate. The laser ablation and surface
modification take place such that the surface of the holding plate
is scanned (sampled) with the focus of the laser irradiation
corresponding to the desired irradiation pattern. The focus of the
laser irradiation is guided in points or lines over the surface of
the ceramic material so that the material is two-dimensionally
removed with the exception of the protrusions. On conventional use
of laser ablation, a depression is made in a solid body surface
which is delimited laterally by solid body material. In contrast to
this conventional material removal limited to points or lines, with
the method according to the invention the material removal
preferably takes place across the surface, wherein the protrusions
remain as freestanding structures. Advantageously, the protrusions
are not adversely affected by the laser ablation and, because of
the gentle material machining, are formed with a higher break
strength than in conventional methods. Particularly preferably, the
material is two-dimensionally removed--with the exception of the
protrusions--with an aspect ratio such that the total area of the
end faces comprises less than 10%, in particular less than 5% or
even less than 3% of the total surface area of the holding plate
which is exposed to the laser irradiation.
Particular advantages for the precise and reproducible setting of
the irradiation pattern on each irradiation step result if,
according to a further preferred embodiment of the invention, the
laser irradiation is generated with a laser source which is moved
relative to the holding plate with a carrier head which can be
moved along at least three, particularly preferably along five
axes. Preferably, the laser source comprises a pulse laser which is
attached directly to the carrier head and is set with this relative
to the holding plate. Alternatively, the laser source comprises a
stationarily positioned pulse laser with a light guide, the output
end of which with a focusing optic is attached to the carrier head
and can be set with this relative to the holding plate. According
to a further alternative, the laser irradiation may be set relative
to the holding plate with pivotable mirrors.
If the maximum working area of the carrier head is smaller than the
lateral extension of the holding plate, the material removal
preferably takes place successively in a plurality of adjacent
surface regions of the holding plate. To irradiate each surface
region, the carrier head and the holding plate are moved relative
to each other such that the respective surface region is covered by
the working area of the carrier head. Particularly preferably, the
laser irradiation is controlled such that patterns of the
arrangement of protrusions continue uninterruptedly over adjacent
surface regions. In other words, the irradiation pattern is
selected in each surface region such that the irradiation pattern
in a first surface region is continued uninterruptedly in the
adjacent surface region. The uninterrupted continuation of the
irradiation pattern means that the position and extent of the
individual surface regions are no longer detectable visually in the
global arrangement of the protrusions.
Preferably, the carrier head and the holding plate are moved
relative to each other between the irradiations of the surface
regions, in particular during the surface modification, such that
the surface regions overlap. Advantageously, this further improves
the homogeneity of the visually perceptible appearance of the
holding plate.
Advantageously, the application of the invention is not restricted
to certain ceramic materials. Rather, the holding plate may be made
of a ceramic material which is selected as a function of the
concrete application of the holding apparatus. Preferred ceramic
materials include SiC, SiSiC, Si.sub.3N.sub.4, CrN, WC, B.sub.4C,
AlN or Al.sub.2O.sub.3. If the ceramic material comprises
Si.sub.3N.sub.4, SiC or SiSiC, for the surface modification
according to the invention, the irradiation parameters of the laser
irradiation are set such that the surfaces of the protrusions have
a higher Si proportion than the volume material. For the other
examples of ceramic materials--in each case
correspondingly--preferably the Cr, W, B or Al phase on the surface
of the protrusions is enriched.
A further advantage of the application of laser ablation according
to the invention in the surface structuring of the holding plate
for a holding apparatus, lies in the substantially greater
variability in the selection of positions, form and size of the
protrusions. The protrusions comprise e.g. columnar protrusions
(so-called burls) or linear protrusions such as webs or ribs. All
protrusions of a holding plate may have the same size and form.
Alternatively, the protrusions may be formed with variable
dimensions and/or forms along the surface of the holding plate. For
example, the burls may be formed with a diameter which increases
from the middle of the holding plate towards its rim.
Advantageously, in this way the burls may have an increased break
strength towards the edge and better absorb the eventually greater
forces occurring at the rim. Alternatively or additionally, the
density of the burls may vary along the surface of the holding
plate. For example, the density of the burls may increase from the
center of the holding plate towards its rim.
For practical applications of the holding plate according to the
invention, it has proved advantageous if the burls are formed with
at least one of the following features. The burls may have a
cylindrical form or a truncated pyramid form or a frustoconical
form. Preferably, the end faces of the burls have a diameter of
less than 500 .mu.m, in particular less than 300 .mu.m,
particularly preferably less than 200 .mu.m. Furthermore, the burls
preferably have a height (axial direction perpendicular to the
lateral extension of the holding plate) which is greater than 25
.mu.m, particularly preferably greater than 150 .mu.m. Particular
advantages for the dissipation of mechanical stresses result if the
burls have a cylindrical shape with a base portion having a rounded
contour.
A further advantage of the invention lies in the variability in the
selection of irradiation parameters of the laser irradiation.
Preferably, the laser irradiation comprises laser pulses with a
pulse duration in the range from 2 ns to 300 ns. Furthermore, the
repetition rate of the laser pulses is preferably selected in the
range from 30 kHz to 200 kHz. The pulse duration and the repetition
rate determine the energy input into the ceramic material.
Advantageously, the pulse duration and/or the repetition rate of
the laser pulses may vary during the material removal, in
particular in the course of the successive irradiation steps. For
example, the material removal may be maximized in a first phase of
the laser ablation, and the formation of a smooth surface of the
burls may be optimized in a second phase of the laser ablation.
The wavelength of the laser irradiation is preferably selected as a
function of the absorption of the ceramic material. If the holding
plate is made from SiSiC, the laser irradiation preferably has a
wavelength at which the absorptions of Si and SiC overlap to a
maximum. In this case, the wavelength is preferably selected in the
range from 500 nm to 1500 nm, particularly preferably in the range
from 900 nm to 1100 nm.
According to a further advantageous variant of the invention, the
laser ablation may be combined with another process of material
removal, in particular a mechanically acting machining or
electro-erosion machining. According to a variant, after the laser
ablation and surface modification of the protrusions, a volume
material between the protrusions may also be removed by means of a
mechanically acting tool and/or by means of sink erosion.
Advantageously, this can increase the speed of surface structuring
of the holding plate.
BRIEF DESCRIPTION OF THE DRAWINGS
Further details of the invention are described below with reference
to the enclosed drawings. The drawings show in:
FIGS. 1 and 2: schematic illustrations of features of the method
according to the invention;
FIG. 3: a schematic top view of the holding apparatus according to
an embodiment of the invention;
FIG. 4: electron-microscopic images of burls produced with a
conventional process and with the method according to the
invention;
FIG. 5: a schematic sectional view of a burl with rounded edges;
and
FIGS. 6 to 7: illustrations of the combination of laser ablation
with an electro-erosion or mechanically acting tools.
Features of preferred embodiments of the invention are described
below, in particular with reference to the manufacturing of a
holding plate with protrusions. Further method steps of
manufacturing of the holding apparatus, such as for example the
combination with further mechanical, electrical and/or pneumatic
components, are not explained since these are known from the
manufacturing of conventional holding apparatuses (clamps).
Preferentially, reference is made below to the formation of burls
with a circular round cross-section and a cylindrical or
frustoconical form (FIGS. 1 to 6). The application of the invention
is not restricted to the formation of these particular forms, but
is also suitable for providing other forms and structures by the
selection of a suitable irradiation pattern of the laser ablation
and/or by combination with material-removal methods (FIG. 7). For
example, burls with a cross-sectional form in the shape of an
ellipse or a rectangle with rounded corners, webs or walls with
straight or curved wall surfaces, can be formed with the use of
laser ablation according to the invention. Although the
manufacturing of protrusions is described only on one surface of
the holding plate, alternatively the protrusions may be formed on
both surfaces of the holding plate.
FIG. 1 illustrates in a schematic side view a laser ablation
machine 20 which is configured for surface structuring of a holding
plate 11 of an electrostatic holding apparatus by means of laser
ablation. The laser ablation machine 20 comprises a laser source
21, a five-axis carrier head 22 and a carrier platform 23 for
receiving the holding plate 11. An operating and control unit 24 is
provided for inputting control data and for controlling the laser
source 21 and the carrier head 22. The carrier head 22 can be moved
with five degrees of freedom which comprise two translational
degrees of freedom in a plane perpendicular to the drawing plane,
and three rotational degrees of freedom corresponding to the three
spatial directions. Further details of the laser ablation machine
20, such as for example a protective housing or auxiliary tools,
are not shown in FIG. 1.
The laser source 21 comprises for example a pulsed Yb fiber laser
with an output power of 20 W, 30 W or 50 W. The laser source 21 is
configured to generate laser pulses with a mean wavelength of 1060
nm, a pulse duration in the range from 2 ns to 200 ns, and a
repetition frequency in the range of 30 kHz and 200 kHz.
The holding plate 11 is for example a circular round disc made from
SiSiC using known sintering methods, and has a plane surface 12.
The diameter of the holding plate 11 lies for example in the range
from 30 cm to 45 cm, and the thickness lies for example in the
range from 1.5 mm to 50 mm. If the holding plate 11 is provided to
hold a component by means of vacuum, it comprises holes for
generating a reduced pressure under a component held on the
protrusions, relative to the external atmospheric pressure. The
blank of the holding plate 11 with the unmachined surface 12 is
fixed onto the carrier platform 23.
The desired process properties for surface structuring of the
holding plate 11 are entered on the input and control unit 24. The
process properties comprise the arrangement and shape of burls to
be formed on the surface 12 by laser ablation, irradiation
parameters for operating the laser source 21, and scanner
parameters for operating the carrier head 22. The irradiation
parameters comprise the choice of pulse duration and repetition
rate for the individual irradiation steps. The scanner parameters
comprise a scanner speed (speed of the carrier head 22 along the
five axes) and--if the maximum working area of the carrier head is
smaller than the extension of the surface 12 of the holding plate
11--the definition of surface regions to be successively subjected
to laser ablation.
On the basis of the desired arrangement and shape of the burls, at
least one irradiation pattern for controlling the carrier head 22
is produced with the input and control unit 24. One single
irradiation pattern is sufficient if the desired height of the
burls can be achieved by material removal in a single irradiation
step, i.e. by a single sweep of the focused laser beam over the
surface 12. Furthermore, one single irradiation pattern is
sufficient if several irradiation steps are provided each with the
same irradiation pattern, for example for producing cylindrical
burls. In the case of laser ablation with several irradiation
steps, a plurality of irradiation patterns may be used in order to
produce the burls with a predefined lateral contour, for example in
the form of a truncated cone or cylinder with a rounded base
region.
The formation of burls 13 on the holding plate 11 in several
irradiation steps is illustrated schematically in FIG. 2. To form
the burls 13 with a height in the range from 50 .mu.m to 200 .mu.m,
at least five irradiation steps are provided followed by the
surface modification of the burls 13 (finishing) according to the
invention. For each of the irradiation steps, the laser irradiation
1 removes a layer with a thickness in the range for example from 10
.mu.m to 20 .mu.m (for example, four intermediate states are
depicted with dotted lines). For this, in the machining of SiSiC
for example, a scanner speed in the range from 700 mm/s to 900
mm/s, a pulse duration of 200 ns and a repetition rate of 30 kHz
are set. The burls 13 are formed with a diameter in the range of
e.g. 50 .mu.m to 500 .mu.m.
The final surface modification takes place with the same scanner
speed and repetition rate of the laser pulses as the material
removal, and with a shorter pulse duration, for example 30 ns. The
surface modification comprises a final irradiation step in which a
change is obtained in the stoichiometric ratios of Si and C on the
surface of the burls 13, in particular on their lateral surfaces
13.2, 13.3, 13.4 (see FIG. 5). Optionally, a slight material
removal may occur. The material removal for surface modification
may e.g. amount to 1/5 to 1/2 of the material removal achieved in
the preceding irradiation steps, e.g. 4 .mu.m to 5 .mu.m. The
stoichiometric ratios are for example changed such that the burl
surfaces primarily comprise Si, so that the burls shine
metallically.
The optimal irradiation parameters and scanner parameters may be
determined, depending on the ceramic material to be machined, using
reference tables or by preliminary experiments. In the case of a
preliminary experiment, a reference plate made from the same
ceramic material as the holding plate 11 to be machined is
subjected to laser ablation, which is carried out in several
reference regions with various irradiation and scanner parameters.
A microscopic examination of the reference regions with visual
assessment of the surface properties achieved gives the desired
irradiation and scanner parameters of manufacturing of the burls 13
and for their surface modification.
After the surface modification of the burls 13, the holding plate
11 with the structured surface may be subjected to post-processing,
for example a cleaning of the structured surface, deposition of an
electrode layer and/or deposition of a dielectric layer. Then the
complete holding apparatus is assembled in that the holding plate
11 is connected to further mechanical, electrical and/or pneumatic
components.
FIG. 3 illustrates schematically a top view of a holding apparatus
10 according to the invention. The holding apparatus 10, which for
example forms a clamp for semiconductor wafers, comprises the
exposed holding plate 11 with the burls 13 and further components
14 illustrated schematically. The holding apparatus 10, apart from
the holding plate 11, is constructed as a conventional holding
apparatus. The burls 13 are arranged according to the selected
irradiation pattern with a predefined geometric distribution. For
example 10,000 to 40,000 burls 13 are provided on the holding plate
11, each with a diameter of 50 .mu.m to 500 .mu.m. It is a
particular advantage of the invention that the laser ablation takes
place so gently and the burls 13 are formed with such a break
strength that even when post-processing is carried out, no burls
are broken away from the finished holding plate 11.
FIG. 4 illustrates a further important advantage of the invention.
FIGS. 4A and 4B show respectively scanning electron microscopic top
views of burls which were produced by conventional sink erosion
(FIG. 4A) and by the laser ablation used according to the invention
with surface modification (FIG. 4B). It is clear that the burls 13
produced according to the invention have a smooth surface which is
free from pores and cracks, in particular in the region of the edge
13.2. This substantially improved surface quality of the burls
produced according to the invention gives the advantages of an
increased mechanical stability due to lesser surface damage
(micro-fissures), a reduced particle emission by abrasion, and in
increased homogeneity of the burl diameter and the circular form of
the burl.
X-ray examinations of the burls shown in FIG. 4 give an Si
proportion of around 15 vol. % for the volume material. A reduced
Si proportion of down to 10 vol. % was found on the surface of the
burls produced by conventional sink erosion (FIG. 4A), whereas in
the burl produced by laser ablation (4B), an increase was found in
the region of 25 vol. % to 45 vol. %. This increase in the Si
proportion due to the method according to the invention, in
particular due to the surface modification with reduced material
removal, offers particular advantages for a reduced particle
emission when the holding apparatus is used for holding
semiconductor wafers. Due to the surface machining, the burl
surface down to a depth of e.g. 30 .mu.m has a greater ductility
than the volume material. The increased ductility contributes to
improving the break strength, since under mechanical loading the
burl surface has a greater tendency to plastic deformation than the
volume material. The plastic deformation allows the absorption of
energy, which reduces the stress on the burl.
FIG. 5 shows a scanning electron microscopic sectional view of a
burl 13 according to a further embodiment of the invention. The
burl 13 has the form of a truncated cone with a flat end face 13.1,
a rounded edge 13.2, a frustoconical side face 13.3 and a rounded
base face 13.4 between the side face 13.3 and the bottom 15 between
adjacent burls. Due to the use of laser ablation according to the
invention, the radii of curvature R.sub.1 and R.sub.2 (with
R.sub.2>R.sub.1) can advantageously be set such that mechanical
stresses on the edge 13.2 of the end face 13.1 or the base face
13.4 are dissipated. The sloping form of the frustoconical lateral
surface is achieved in that the irradiation pattern is varied
slightly on each irradiation step (stepped reduction in irradiation
region between the burls 13 with increasing material removal). By
deviation from FIG. 5, a burl 13 may be formed with a cylindrical
lateral surface, wherein the laser source 21 with a carrier head 22
movable in five axes is operated with an irradiation direction
sloping towards the burl 13.
Experimental investigation of the roughness of the burl surface at
various irradiation and scanner parameters showed a minimum
roughness from a repetition rate of around 100 kHz, pulse durations
of 30 ns and 100 ns, and various scanning speeds.
FIGS. 6 and 7 illustrate schematically different applications of
the combination of laser ablation with other methods of material
removal. According to FIG. 6, laser ablation is used to form the
burls 13 in the surface 12 of the holding plate 11 (FIGS. 6A, B).
The laser irradiation 1 is controlled such that the ceramic
material is removed only in the immediate vicinity of the burls 13
and subjected to the surface modification according to the
invention. The volume material 16 remaining in the gaps between the
burls 13 (shown in hatching in FIG. 6C) is then removed by sink
erosion with increased removal speed. Since the sink erosion is
restricted to the regions between the burls 13, the disadvantages
of conventional usage of sink erosion for the burls 13 are avoided.
Furthermore, the removal speed is substantially increased.
Alternatively, the laser ablation may be combined with a machining
removal process as shown in FIG. 7. This is particularly
advantageous for the structuring of ceramic materials which are
poorly suited to erosion processing, such as for example SiN, AlN
or Al.sub.2O.sub.3. In a first removal step, by laser ablation by
means of the laser irradiation 1, the burls 13 are exposed and
subjected to surface modification (FIGS. 7A, 7B), wherein said
advantages in relation to the surface, break strength and shape of
the burls 13 are exploited. Then using a material-removal tool,
such as for example an ultrasonic grinding tool 30, the volume
material 16 between the burls 13 is removed (FIG. 7C).
The features of the invention disclosed in the description above,
the drawings and the claims may be important both individually and
in combination or sub-combination for implementing the invention in
its various embodiments.
* * * * *
References